This is a U.S. national stage of application No. PCT/DE98103753, filed on Dec. 21, 1998. Priority is claimed on that application:
Country: Germany, Application No.: 197 57 689.3, Filed: Dec. 23, 1997.
Field of the Invention and Discussion of the Prior Art
The invention relates first of all to an inductive measurement transducer. An inductive measurement transducer has a stator and a carriage which can be moved with respect to it, having an inductive transmitter element and associated evaluation electronics, whose output signal represents a measure of the position of the carriage relative to the stator. The stator has a field coil which extends over the measurement distance and is fed with alternating current, and whose magnetic field passes through a transmitter element which is connected to the carriage and is in the form of a soft-magnetic core and/or a coil, and in turn passes through a secondary winding connected to the stator, where it induces a voltage dependent on the position of the carriage. This secondary winding consists of one turn, from which voltage elements are obtained by appropriate taps distributed over the measurement distance. The voltage elements are averaged to produce a voltage which is dependent on the position of the carriage. The averaging can be carried by resistors or else by other electrical components. Instead of discrete components, resistance layers or capacitor plates may also be used. In one particularly advantageous embodiment, the field winding consists of a single turn, with the housing and guides being used as the coil. A further improvement is obtained by designing the transmitter element as a tuned circuit, and this greatly reduces the formation of stray fields. In this case, it is advantageous for the sensor to be operated at the resonant frequency of this tuned circuit, by using the tuned circuit as the element which defines the frequency of an oscillator.
The invention furthermore relates to an arrangement for measuring angles.
The advantage of inductive distance/angle sensors is a low sensitivity to dirt and other environmental influences. Differential inductors are generally known, in which a soft-magnetic core is positioned in two coils and its position influences the inductance of the two coils such that a voltage which is dependent on the position can be tapped off between the two coils, which are connected in series and are connected to an AC voltage. This configuration has the disadvantage that the physical length of the coil amounts to at least twice the measurement distance and, furthermore, the mechanical connection of the coil also projects beyond the measurement distance in the final position, so that an installation length of at least three times the measurement distance is required. In addition, the measurement result is influenced by the temperature response of the winding resistance and the permeability of the magnet material used.
In addition, solutions have been proposed in which either short-circuit cylinders or cores of high permeability are inserted into a coil, with the change in the inductance of the coil being used as a measure of the position of the core. These solutions have the advantage over differential transformers that their physical length is shorter. However, when the measurement rod is extended, at least twice the measurement distance is still required. Furthermore, the temperature response of the winding resistance and of the core has a more pronounced effect on the temperature characteristic.
From German reference DE-A-2511683, it is known for a suitable measurement signal to be produced by injecting a voltage from a field winding via a high-permeability core into a measurement winding with an increasing turns density. This method has the advantage of a short physical length, but has the disadvantage that the distribution of the magnetic field over the area of the measurement coil and the configuration of the measurement coil are subject to stringent requirements.
Furthermore, an embodiment is known from WO 94/03778 in which a reduction in the voltage drop within part of the region of the coil is produced by means of a short-circuiting ring on a coil, and a signal which is dependent on the position of the short-circuiting ring is formed by means of resistors or capacitors from taps on the coil. This results in a sensor having a short physical length. The disadvantageous feature is the influence of the winding resistance and of the stray inductances.
The object of the invention is to overcome the disadvantages of the apparatuses mentioned above and to propose a distance sensor which can be constructed using simple means and is highly accurate.